In a wireless communication system, a UE's Power Headroom Report (PHR) provides the UE's power information to a base station for the base station to schedule, for example, an appropriate uplink resources and modulation and/or coding scheme for an uplink transmission by the UE. Conventionally, there is only one serving base station for a UE, such that the UE may only need to send the PHR of the active serving cells to the base station. When two or more links are supported, the UE may need to send PHRs to two or more serving base stations.
For example, in a long term evolution (LTE) Dual Connectivity (DC) scenario, power headroom of all active serving cells with configured uplink should be reported in a PHR Medium Access Control (MAC) Control Element (CE) field to the serving base stations. In order to avoid waste of resources, a base station (e.g., an evolved NodeB (eNB)) involved in the LTE DC scenario should know the power limitation of the corresponding UE. Specifically, to assist the eNB to schedule the uplink transmission resources for the UE in an appropriate way, the UE may need to be able to report its available power headroom to the eNB. The eNB may then use the received power headroom report to determine how much more uplink bandwidth (e.g., per sub-frame) the UE is capable of using.
In a next generation (e.g., 5th Generation New Radio (5G NR) wireless communication network, similar principles may apply for the same purpose (e.g., for an eNB and a next generation nodeB (gNB) to negotiate the maximum allowed power values for LTE (P_LTE) and NR (P_NR)). However, whether and/or how one Radio Access Technology (RAT) (e.g., LTE) is able to know about or understand the PHR of another RAT (e.g., NR) has not been extensively discussed. In addition, in order to reuse the Dual Connectivity PHR MAC CE for an Evolved-Universal Terrestrial Radio Access (UTRAN) Network New Radio-Dual Connectivity (EN-DC) scenario, there is no particular method of reporting a supplemental uplink (SUL), that is associated with a serving cell, without significantly changing the existing PHR MAC CE structure, which is designed based on a per cell basis, and not a per uplink carrier basis. The existing PHR MAC CE structure, which supports at most 32 serving cells, makes it difficult for EN-DC and a Multiple Entry (ME) PHR MAC CE, to consider supporting a SUL carrier for all serving cells, especially when a master eNB (MeNB) and a secondary gNB (SgNB) are not aware of each other's configurations. Specifically, a master eNB (MeNB) may not know which serving cell in a secondary cell group (SCG) is configured with a SUL.
In NR, the PH field may have 6 bits, similar to the LTE PH field. That is, NR may support a PHR format including a bitmap, a type 2 PH field for the Special Cell (SpCell) of the other MAC entity, a type 1 PH field for the Primary Cell (PCell), and one or more type X PH fields for the serving cells in an ascending order according to the Serving Cell Index (ServCellIndex), where X is either 1 or 3 based on the configurations or predefined rules. It should be noted that in a Dual Connectivity operation, the term Special Cell (SpCell) refers to the PCell of the MCG or the PSCell of the SCG, otherwise the term Special Cell refers to the PCell. In an MR-DC (e.g., in an EN-DC) case, the master node (MN) provides the secondary node (SN) with the range of Cell Index (SCellIndex or ServCellIndex) to be used by signaling a start value and a stop value if cell index range for 32 cells is used between the MN and the SN. The UE may use Dual Connectivity PHR MAC CE supporting up to 32 serving cells with configured uplink for the PH reporting to eNB. On the other hand, the UE may use Multiple Entry (ME) PHR MAC CE supporting up to 32 serving cells with configured uplink for the PH reporting to gNB.
In addition, in the LTE-NR scenario discussed above, SULs may be used in NR. The SUL carriers may help resolve potential uplink coverage issues. For example, when a UE initiates an initial access while camping on an SUL-capable cell, an SUL carrier selection operation is performed based on a threshold configured in the Remaining Minimum System Information (RMSI). This allows the UE to select the SUL carrier if, and only if, the Received Signal Strength Indicator (RSRP) on the Downlink (DL) carrier is not satisfied compared to the threshold in order to enable the Random Access (RA) procedure by the assistance of the SUL carrier.
Moreover, for a contention based RA procedure, if the network does not explicitly tell the UE which carrier to use, the UE may perform an Uplink (UL) selection based on the RSRP threshold as an initial access. In an EN-DC case, the Master Cell Group (MCG) carriers include LTE frequencies for both UL and DL. Therefore, there is no use case for the MCG to configure the SUL. For the EN-DC Secondary Cell group (SCG), however, it is yet to be determined whether the SUL carrier is used for the PSCell only, or for all serving cells (including the PSCell) of the NR SCG. Using the SUL for the contention-free Random Access Channel (RACH) procedure in a serving cell may also improve the successful rate of Contention-Free Random Access (CFRA) in the serving cell.